SpringBoot-Web 静态资源映射规则

SpringBoot当然可以存放页面、图片等静态资源，但这些静态资源放在哪个文件中，又如何去访问它们呢？可以用源码来解读：
先进入WebMvcAutoConfiguration类中，这个类包含了SpringMVC的web配置，其中有一个addResourceHandlers()方法：

        public void addResourceHandlers(ResourceHandlerRegistry registry) {
            if (!this.resourceProperties.isAddMappings()) {
                logger.debug("Default resource handling disabled");
            } else {
                Duration cachePeriod = this.resourceProperties.getCache().getPeriod();
                CacheControl cacheControl = this.resourceProperties.getCache().getCachecontrol().toHttpCacheControl();
                if (!registry.hasMappingForPattern("/webjars/**")) {
                    this.customizeResourceHandlerRegistration(registry.addResourceHandler(new String[]{"/webjars/**"}).addResourceLocations(new String[]{"classpath:/META-INF/resources/webjars/"}).setCachePeriod(this.getSeconds(cachePeriod)).setCacheControl(cacheControl));
                }

                String staticPathPattern = this.mvcProperties.getStaticPathPattern();
                if (!registry.hasMappingForPattern(staticPathPattern)) {
                    this.customizeResourceHandlerRegistration(registry.addResourceHandler(new String[]{staticPathPattern}).addResourceLocations(WebMvcAutoConfiguration.getResourceLocations(this.resourceProperties.getStaticLocations())).setCachePeriod(this.getSeconds(cachePeriod)).setCacheControl(cacheControl));
                }
            }
        }

可以看到首先调方法isAddMappings()判断是否做了新增映射，点进这个方法：

public boolean isAddMappings() {
    return this.addMappings;
}

这个方法属于ResourceProperties类，在看这个类的属性与构造：

    private static final String[] CLASSPATH_RESOURCE_LOCATIONS = new String[]{"classpath:/META-INF/resources/", "classpath:/resources/", "classpath:/static/", "classpath:/public/"};
    private String[] staticLocations;
    private boolean addMappings;
    private final ResourceProperties.Chain chain;
    private final ResourceProperties.Cache cache;

    public ResourceProperties() {
        this.staticLocations = CLASSPATH_RESOURCE_LOCATIONS;
        this.addMappings = true;
        this.chain = new ResourceProperties.Chain();
        this.cache = new ResourceProperties.Cache();
    }

发现在构造ResourceProperties类时addMappings属性就已经置为了true，所以之前WebMvcAutoConfiguration类的addResourceHandlers方法直接进入else代码块，并且registry调用addResourceHandler()新增了一个资源处理器，所有的/webjars/**都要去classpath:/META-INF/resources/webjars/这个路径下找资源，webjars本质就是以jar包的方式引入静态资源，可以引入
webjars中的jquery依赖：

<dependency>
    <groupId>org.webjars</groupId>
    <artifactId>jquery</artifactId>
    <version>3.3.1</version>
</dependency>

导入依赖后发现在外部包中存在：

再回到ResourceProperties类中，ResourceProperties类中的第一个属性CLASSPATH_RESOURCE_LOCATIONS：

private static final String[] CLASSPATH_RESOURCE_LOCATIONS = new String[]{"classpath:/META-INF/resources/", "classpath:/resources/", "classpath:/static/", "classpath:/public/"};

与ResourceProperties类中appendSlashIfNecessary()方法：

    private String[] appendSlashIfNecessary(String[] staticLocations) {
        String[] normalized = new String[staticLocations.length];

        for(int i = 0; i < staticLocations.length; ++i) {
            String location = staticLocations[i];
            normalized[i] = location.endsWith("/") ? location : location + "/";
        }

        return normalized;
    }

表明有五个路径指向可以存放静态资源的位置，第五个是项目根目录下：

1. “classpath:/META-INF/resources/”
2. “classpath:/resources/”
3. “classpath:/static/”
4. “classpath:/public/”
5. “/”

而且访问优先级先从 classpath:/resources/ ==> classpath:/static/ ==> “classpath:/public/”
静态资源的存放路径清楚后，可以看到WebMvcAutoConfiguration类中有welcomePageHandlerMapping()欢迎页面的映射方法：

@Bean
public WelcomePageHandlerMapping welcomePageHandlerMapping(ApplicationContext applicationContext, FormattingConversionService mvcConversionService, ResourceUrlProvider mvcResourceUrlProvider) {
    WelcomePageHandlerMapping welcomePageHandlerMapping = new WelcomePageHandlerMapping(new TemplateAvailabilityProviders(applicationContext), applicationContext, this.getWelcomePage(), this.mvcProperties.getStaticPathPattern());
    welcomePageHandlerMapping.setInterceptors(this.getInterceptors(mvcConversionService, mvcResourceUrlProvider));
return welcomePageHandlerMapping;
}

里面有一个getWelcomePage()方法，再点进去：

private Optional<Resource> getWelcomePage() {
    String[] locations = WebMvcAutoConfiguration.getResourceLocations(this.resourceProperties.getStaticLocations());
    return Arrays.stream(locations).map(this::getIndexHtml).filter(this::isReadable).findFirst();
}

private Resource getIndexHtml(String location) {
    return this.resourceLoader.getResource(location + "index.html");
}

getWelcomePage()方法当中调用了getIndexHtml()方法，获取index页面，可得出结论：静态资源下的所有index页面被/**所映射，在浏览器访问localhost:8080/后会直接跳转到静态文件目录下的index.html。

如果在全局配置当中配置了：

spring.resources.static-locations=classpath:xxx

默认的配置全部失效，只认自己配置的静态资源目录。